Thin film transistor-liquid crystal display having enhanced storage capacitance and method for manufacturing the same

ABSTRACT

Disclosed are a thin film transistor-liquid crystal display(TFT-LCD) improving aperture ratio and having increased storage capacitance, and a manufacturing method of the same. 
     The TFT-LCD comprises: a glass substrate; gate bus lines arranged parallel each other on the glass substrate; data bus lines disposed perpendicular to the gate bus lines thereby defining pixel region; a thin film transistor formed adjacent to each intersection of the gate bus line and the data bus line; a transparent pixel electrode being contacted with the thin film transistor and disposed at each pixel region; a transparent storage electrode formed at a bottom of the transparent pixel electrode, wherein the transparent storage electrode forms a storage capacitance together with the pixel electrode; and a common electrode line for transmitting common signal to the storage electrode.

FIELD OF THE INVENTION

The present invention generally relates to a liquid crystal display,more particularly to a thin film transistor-liquid crystaldisplay(“TFT-LCD”) and method for manufacturing the same.

BACKGROUND OF THE INVENTION

The active matrix type LCD employing .a thin film transistor as a meansfor activating and having a number of pixels, has a thin and lightdevice size and displays excellent picture quality comparable to theCathode Ray Tube monitor.

There are two types of active matrix LCD device depending on thelocation of storage electrode, i.e. the storage-on-gate type and thestorage-on-common type.

At first, referring to FIG. 1, general active matrix type LCD is shown.

A gate bus line 2 and a data bus line 4 are arranged in a matrixconfiguration thereby defining pixel region. The gate bus line 2includes a storage electrode 3 being extruded to an outer side of itscorresponding pixel. More preferably, a storage capacitance electrode 3is disposed at a previous pixel.

A thin film transistor 10 is disposed adjacent to an intersection of thegate bus line 2 and the data bus line 4. The thin film transistor 10includes a gate electrode 2 a being extended from the gate bus line 2 toits corresponding pixel; a channel layer 5 disposed on the gateelectrode 2 a; a source electrode 4 a being extended from the data busline 4 and in contact with one side of the channel layer 5; and a drainelectrode 4 b being contacted with the other side of the channel layer5.

A pixel electrode 7 is made of a transparent film, for example, theITO(indium tin oxide). Further, the pixel electrode 7 is overlapped withthe storage electrode 3 thereby forming a storage capacitance Cst.

In the meantime, the constitutions of active matrix LCD of generalstorage-on-common type will be discussed with reference to FIG. 2.

Referring to FIG. 2, a gate bus line 2 and a data bus line 4 arearranged in the matrix configuration thereby defining pixel region. Athin film transistor 10 is disposed adjacent to an intersection of thegate bus line 2 and the data bus line 4.

A storage electrode 6 is disposed parallel with the gate bus line 2 andis formed between a pair of gate bus lines 2.

A pixel electrode 7 is made of the ITO, and is formed one per pixel. Thepixel electrode 7 is overlapped with the storage electrode 6 therebyforming a storage capacitance Cst. At this time, the storage electrode 6has a larger dimension at some region thereof that is overlapped withthe pixel electrode compared to the other region that is not overlappedwith the pixel electrode 7 to form an adequate amount of storagecapacitance.

However, those LCD devices of storage-on-gate type and storage-on-commontype have following shortcomings.

First of all, in an LCD of the storage-on-gate type, as the dimension ofgate bus line 2 is increased, the RC time constant in the gate bus line2 is increased. Accordingly, signal delay of the gate bus line 2 isincreased.

On the other hand, in an LCD of the storage-on-common type, as thestorage electrode 6 is disposed with a selected dimension in the pixelregion, the aperture ratio is decreased. Furthermore, there is formed aregion intersected by the data us line. It is also involves a risk ofdisconnection.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide aTFT-LCD capable of reducing the signal delay in the gate-bus-line.

It is another object of the present invention to provide a TFT-LCDcapable of improving the aperture ratio.

It is further object of the present invention to provide a TFT-LCDhaving enhanced storage capacitance.

To accomplish the objects of the present invention, in one aspect, thepresent invention provides a TFT-LCD comprising: a glass substrate; gatebus lines arranged parallel each other on the glass substrate; data buslines disposed perpendicular to the gate bus lines thereby definingpixel region; a thin film transistor formed adjacent to eachintersection of the gate bus line and the data bus line; a transparentpixel electrode being contacted with the thin film transistor anddisposed at each pixel region; a transparent storage electrode formed ata bottom of the transparent pixel electrode, wherein the transparentstorage electrode forms a storage capacitance together with the pixelelectrode; and a common electrode line for transmitting common signal tothe storage electrode.

The present invention further provides a TFT-LCD comprising: a glasssubstrate; gate bus lines arranged parallel each other on the glasssubstrate; data bus lines disposed perpendicular to the gate bus linesthereby defining pixel region; a thin film transistor formed adjacent toeach intersection of the gate bus line and the data bus line; atransparent pixel electrode being contacted with the thin filmtransistor and disposed at each pixel region; a transparent storageelectrode formed at a bottom of the transparent pixel electrode, whereinthe transparent storage electrode forms a storage capacitance togetherwith the pixel electrode; and a common electrode line for transmittingcommon signal to the storage electrode, wherein the storage electrodehas the same size and shape with the pixel electrode, wherein the commonelectrode line has the minimum line width preventing signal delay.

In another aspect, the present invention provides a method comprisingthe steps of: depositing an opaque metal film on a glass substrate;forming gate bus lines and common electrode lines by patterning aselected portion of the opaque metal film; depositing an ITO layer onthe glass substrate; forming a storage electrode by patterning aselected portion of the ITO layer so as to be contacted with the commonelectrode line; forming a gate insulating layer on the glass substratein which the storage electrode is formed; and forming a pixel electrodeon the gate insulating layer so that the entire pixel electrode regionis overlapped with the storage electrode.

The present invention further provides a method comprising the steps of:depositing an ITO layer on a glass substrate; forming a storageelectrode by patterning a selected portion of the ITO layer; depositingan opaque metal film on the glass substrate in which the ITO layer isformed; forming gate bus lines and common electrode sines by patterninga selected portion of the opaque metal film, wherein the commonelectrode lines are contacted with the storage electrode; forming a gateinsulating layer on a resultant of the glass substrate; and forming apixel electrode on the gate insulating layer so that the entire pixelelectrode region is overlapped with the storage electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing an active matrix LCD of a generalstorage-on-gate type.

FIG. 2 is a plan view showing an active matrix LCD of a generalstorage-on-common type.

FIG. 3 is a plan view showing a TFT-LCD according to a first embodimentof the present invention.

FIG. 4 is a cross-sectional view taken along IV-IV′ of FIG. 3 forshowing a TFT-LCD according to a second embodiment of the presentinvention.

FIG. 5 is a cross-sectional view taken along IV-IV′ of FIG. 3 forshowing a TFT-LCD according to a third embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the preferred embodiments of the present invention will bedescribed in detail.

First Embodiment: TFT-LCD

Referring to FIG. 3, a plurality of gate bus lines 21 are arranged at alower glass substrate 20 with a regular distance. A plurality of databus lines 26, are also arranged at the lower glass substrate 20 with aregular distance and disposed to be crossed with the gate bus lines 21thereby defining pixel region.

A thin film transistor 30 is disposed adjacent to an intersection of thegate bus line 21 and the data bus line 21. The thin film transistor 30includes a gate electrode 21 a being extended from the gate bus line 21to the pixel; a channel layer 24 disposed on the gate electrode 21 a; asource electrode 26 a being extended from the data bus line 26 to becontact with one side of the channel layer 24; and a drain electrode, 26b being contacted with the other side of the channel layer 24.

A pixel electrode 27 is disposed at pixel region being surrounded withthe gate bus line 21 and the data bus line 25 so that the pixelelectrode 27 is contacted with the thin film transistor 30. As known inthe art, the pixel electrode 27 is made of a transparent conductor, forexample ITO(indium tin oxide) material.

A storage electrode 29 a is formed at a bottom of the pixel electrode 27so that the storage electrode 29 a is overlapped with the pixelelectrode 27. The storage electrode 29 a is shaped similar to the pixelelectrode 27 and is made of a transparent conductor, such as ITO. Acommon electrode line 29 b is in contact with the storage electrode 29 aand is extended parallel to the gate bus line 21. The common electrodeline 29 b is made of the same material as the gate bus line 21, i.e. anopaque conductor and is formed between a pair of gate bus lines beingdisposed adjacent to the common electrode line 29 b. Further, the commonelectrode lint 29 b acts for transmitting a common signal to the storageelectrode 29 a without forming any storage capacitance with the pixelelectrode 27, therefore the common electrode line 29 b Should have afine line width which is fine enough not to cause signal delay.Accordingly, within a pixel, the common electrode line 29 b of thepresent embodiment has a finer line width than that of the storageelectrode of conventional storage-on-common type. Then, aperturedimension of the LCD device is enlarged.

Moreover, a gate insulating layer(not shown) is formed between the gatebus line 21 and the data bus line 25, and between the storage, electrode29 a and the pixel electrode 27 thereby insulating therebetweenrespectively.

As described above, the storage electrode 29 a is formed to have thesame size with the pixel electrode 27 thereby forming storagecapacitance Cst in the entire pixel electrode 27. Therefore, the storagecapacitance is greatly improved compared to the conventional devices andthe picture quality of LCD device is also enhanced.

More concretely, the pixel electrode variation which relates to thepicture quality closely, can be shown as following function ofcapacitance Cst, i.e. the equation 1. $\begin{matrix}{{\Delta \quad {Vp}} = {\frac{Cgs}{{Cgs} + {Cst} + {Clc}} \times \Delta \quad {Vg}}} & {{equation}\quad 1}\end{matrix}$

wherein, Δ Vp means the variation of pixel voltage,

Δ Vg means the variation of gate signal,

Csg means the parasitic capacitance between the gate electrode and thesource electrode,

Cst means a storage capacitance, and

Clc means a capacitance of a counter electrode and a pixel electrode.

According to the equation 1, the variation of pixel voltage is mainlyaffected by not only the storage capacitance Cst but the parasiticcapacitance Cgs raised between the gate electrode and the sourceelectrode, and the capacitance Clc between the counter electrode and thepixel electrode. Especially, the parasitic capacitance Cgs of the gateand source electrodes is easily changed during the process ofmanufacturing if there are caused a lot of changes due to themisalignment of mask, and the capacitance Clc of the counter and pixelelectrodes is also easily changed due to the dielectric anisotropy ofliquid crystal.

However, if the storage capacitance Cst as in the present embodiment isoutstandingly greater than the parasitic capacitance Cgs of the gate andsource electrodes and the capacitance Clc of the counter and sourceelectrodes, both affect the variation of pixel voltage, then, thevariation of the pixel voltage is not affected so much although theparasitic capacitance Cgs and the capacitance Clc are changed.

Accordingly, an enhanced picture quality is obtainable.

Moreover, since the common electrode line 29 b for transmitting thecommon signal to the storage electrode 29 a is formed with fine linewidth which is the minimum width sufficient to prevent signal delay, theaperture ratio is also improved compared to the conventional one.

Second Embodiment: Method for Manufacturing TFT-LCD

Referring to FIG. 4, an opaque metal film is formed on a glass substrate20. A gate bus line(not shown) and a common electrode line 29 b areformed by patterning some portions of the opaque metal film. The commonelectrode line 29 b is formed between the gate bus lines as in the firstembodiment. An ITO layer is formed on the glass substrate 20 to becontacted with the common electrode line 29 b. Afterward, a storageelectrode 29 a is formed by patterning a selected portion of the ITOlayer. As a result, the storage electrode 29 a is contacted with upperand side portions of the common electrode line 29 b. A gate insulatinglayer 23 is formed on the glass substrate 20 in which a storageelectrode 29 a is formed.

Afterward, although not shown in the drawing, a channel layer(not shown)is formed at a selected portion on the gate bus line and the gateinsulating layer 23 is coated on the ITO layer. Next, the ITO layer isformed in the same size with the storage electrode 29 a thereby forminga pixel electrode 27.

At this time, the entire region of the pixel electrode 27 is overlappedwith the storage electrode 29 a, therefore a storage capacitance isformed all over the pixel electrode region.

Although not shown in the drawing, there are followed subsequent stepsof forming source and drain electrodes and data bus line.

Third Embodiment: Method for Manufacturing TFT-LCD

Referring to FIG. 5, an ITO layer is formed on a glass substrate 20. Astorage electrode 29 a is formed by patterning some portions of the ITOlayer. An opaque metal film is formed on the glass substrate 20 in whichthe storage electrode 29 a is formed. A gate bus line(not shown) and acommon electrode line 29 b are formed by patterning a selected portionof the opaque metal film. Herein, the common electrode line 29 b iscontacted with the storage electrode 29 a and the gate bus line(notshown) is not contacted with the storage electrode 29 a.

Afterward, although not shown, a channel layer(not shown) is formed at aselected portion on the gate bus line and an insulating layer 23 iscoated on the ITO layer. Next, the ITO layer is formed in the same sizewith the storage electrode 29 a thereby forming a pixel electrode 27.

At this time, the entire region of the pixel electrode 27 is overlappedwith the storage electrode 29 a, therefore a storage capacitance isformed all over the pixel electrode region.

Although not shown in the drawing, there are followed subsequent stepsof forming source and drain electrodes and data bus line.

As disclosed in the above specification, according to the presentinvention, since the storage capacitance is formed in the entire pixelelectrode region, the storage capacitance of the TFT-LCD is increased.As a result, the variation of pixel voltage can be reduced and picturequality is also improved.

Further, since the common electrode line for transmitting common signalto the storage electrode has a reduced line width compared to theconventional one, the aperture ratio is also increased compared to theconventional TFT-LCD.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof the present invention.

What is claimed is:
 1. A thin film transistor-liquid crystal display(TFT-LCD) comprising: a glass substrate; a gate bus lines arrangedparallel to each other on the glass substrate; data bus lines disposedperpendicular to the gate bus lines, a pair of the gate bus lines and apair of the data bus lines forming a pixel region; a thin filmtransistor; a transparent pixel electrode connected to the thin filmtransistor and disposed in the pixel region, the pixel electrode havinga first side, which is located proximate to one of the pair of the gatebus lines, a second side, which is located proximate to the other one ofthe pair of the gate bus lines, a third side, which is located proximateto one of the pair of the data bus lines, and a fourth side, which islocated proximate to the other one of the pair of the data bus lines,whereby the pixel electrode spans the substantially entire pixel region;a transparent storage electrode formed at a bottom of the pixelelectrode, wherein the storage electrode spans the entire pixelelectrode such that the storage electrode and the pixel electrode form astorage capacitor along the entire pixel electrode and hence thesubstantially entire pixel region; and a common electrode line fortransmitting a common signal to the storage electrode.
 2. The TFT-LCD ofclaim 1, wherein the storage electrode has the same size and shape withthe pixel electrode.
 3. The TFT-LCD of claim 1, wherein the commonelectrode line is made of the same material with the gate bus line. 4.The TFT-LCD of claim 1, wherein the common electrode line has theminimum line width preventing signal delay.
 5. The TFT-LCD of claim 1,wherein the common electrode line is formed between a pair of gate buslines being disposed adjacent to the common electrode line, and thecommon electrode line is disposed parallel to the gate bus lines.
 6. TheTFT-LCD of claim 1, wherein the pixel electrode and the storageelectrode are made of ITO(indium tin oxide) layers.
 7. The TFT-LCD ofclaim 1, wherein the common electrode line has a portion which is incontact with the storage electrode and which extends from one side ofthe pixel region to an opposite side of the pixel region such that itextends completely across the pixel region.
 8. A TFT-LCD comprising: aglass substrate; gate bus lines arranged parallel to each other on theglass substrate; data bus lines disposed perpendicular to the gate buslines, a pair of the gate bus lines and a pair of the data bus linesforming a pixel region; a thin film transistor; a transparent pixelelectrode connected to the thin film transistor and disposed in thepixel region, the pixel electrode having a first side, which is locatedproximate to one of the pair of the gate bus lines, a second side, whichis located proximate to the other one of the pair of the gate bus lines,a third side, which is located proximate to one of the pair of the databus lines, and a fourth side, which is located proximate to the otherone of the pair of the data bus lines, whereby the pixel electrode spansthe substantially entire pixel region; a transparent storage electrodeformed at a bottom of the pixel electrode, wherein the storage electrodespans the entire pixel electrode such that the storage electrode and thepixel electrode form a storage capacitor along the entire pixelelectrode and hence the substantially entire pixel region; and a commonelectrode line for transmitting a common signal to the storageelectrode, wherein the storage electrode has a size and shape identicalto those of the pixel electrode, wherein the common electrode line has aminimum width for preventing signal delay.
 9. The TFT-LCD of claim 8,wherein the pixel electrode and the storage electrode are made of ITOlayers.
 10. The TFT-LCD of claim 8, wherein the common electrode line ismade of the same material with the gate bus line.
 11. The TFT-LCD ofclaim 8, wherein the common electrode line is formed between a pair ofgate bus lines being disposed adjacent to the common electrode line, andthe common electrode line is disposed parallel to the gate bus lines.12. The TFT-LCD of claim 8, wherein the common electrode line has aportion which is in contact with the storage electrode and which extendsfrom on side of the pixel region to an opposite side of the pixel regionsuch that it extends completely across the pixel region.
 13. A methodfor manufacturing a TFT-LCD, comprising the steps of: depositing anopaque metal film on a glass substrate; forming gate bus lines andcommon electrode lines by patterning a selected portion of the opaquemetal film; depositing an ITO layer on the glass substrate; forming astorage electrode in contact with a corresponding one of the commonelectrode lines by patterning a selected portion of the ITO layer;forming a gate insulating layer on the glass substrate in which thestorage electrode is formed; forming a pixel electrode on the gateinsulating layer; and forming data bus lines, a pair of the gate buslines and a pair of the data bus lines forming a pixel region such thatthe pixel electrode is disposed in the pixel region, the pixel electrodehaving a first side, which is located proximate to one of the pair ofthe gate bus lines, a second side, which is located proximate to theother one of the pair of the gate bus lines, a third side, which islocated proximate to one of the pair of the data bus lines, and a fourthside, which is located proximate to the other one of the pair of thedata bus lines, whereby the pixel electrode spans the substantiallyentire pixel region, and the storage electrode being formed so as tospan the entire pixel electrode such that the storage electrode and thepixel electrode form a storage capacitor along the entire pixelelectrode and hence the substantially entire pixel region.
 14. Themethod of claim 13, wherein the corresponding one of the commonelectrode lines has a portion which is in contact with the storageelectrode and which extends from one side of the pixel region to anopposite side of the pixel region such that it extends completely acrossthe pixel region.
 15. A method for manufacturing a TFT-LCD, comprisingthe steps of: depositing an ITO layer on a glass substrate; forming astorage electrode by patterning a selected portion of the ITO layer;depositing an opaque metal film on the glass substrate in which the ITOlayer is formed; forming gate bus lines and common electrode lines bypatterning a selected portion of the opaque metal film, wherein thestorage electrode is in contact with a corresponding one of the commonelectrode lines; forming a gate insulating layer on a resultant of theglass substrate; forming a pixel electrode on the gate insulating layer;and forming data bus lines, a pair of the gate bus lines and a pair ofthe data bus lines forming a pixel region such that the pixel electrodeis disposed in the pixel region, the pixel electrode having a firstside, which is located proximate to one of the pair of the gate buslines, a second side, which is located proximate to the other one of thepair of the gate bus lines, a third side, which is located proximate toone of the pair of the data bus lines, and a fourth side, which islocated proximate to the other one of the air of the data bus lines,whereby the pixel electrode spans the substantially entire pixel region,and the storage electrode being formed so as to span the entire pixelelectrode such that the storage electrode and the pixel electrode form astorage capacitor along the entire pixel electrode and hence thesubstantially entire pixel region.
 16. The method of claim 15, whereinthe corresponding one of the common electrode lines has a portion whichis in contact with the storage electrode and which extends from one sideof the pixel region to an opposite side of the pixel region such that itextends completely across the pixel region.